Zirconia-boron ablation coating



Jan. 25, 1966 L. E. FULLER ZIHGONIA-BORON ABLATION COATING Filed June 9. 1961 INVENTOR. LYLE E.FULLER United States Patent 3,231,416 ZIRCONIA-BORON ABLATION COATING Lyle E. Fuller, Indianapolis, Ind., assignor to Union Carbide Corporation, a corporation of New York Filed June 9, 1961, Ser. No. 115,912 9 Claims. (Cl. 117105.2)

The present invention relates to a high temperature resistant article; it relates more particularly to an article having an outer coating possessing low thermally conductive characteristics, and being ablatable under severe heating conditions.

There is a present and ever-growing future need in the aeronautic and astronautic industries to provide protection to the outer surface and especially to the thin walled nose section of super high velocity missiles or space vehicles as they re-enter and pass through the earths atmosphere. Air or atmospheric friction, especially during the re-entry period of a space flight, can effect an excessively high temperature build up in the exposed outer surface to cause gross melting or other failure of virtually all presently known structural materials.

One means of protecting such materials involves coating the outer surface of the fuselage and/ or nose section of the high velocity projectile with a layer of material that will resist thermal damage under high heat flux conditions. The protective coating, in order to function properly, should have a high melting point and should also have the desirable properties of high resistance to thermal shock, low transmission of thermal energy through the coating as indicated by a low back face temperature, and relatively low density.

In accordance with the present requirements, high altitude missile components such as the nose cone which is to re-enter the earths atmosphere after outer space travel, are usually, though not necessarily, thin walled and light. The metal employed in the construction of this member may be stainless steel, titanium, aluminum, or similar high temperature resistant air craft type materials. It has been found though that even when provided with special shapes to facilitate re-entry into the earths atmosphere, such members, unless adequately cooled or heat shielded will literally burn up and be completely consumed in a very short time due to the amount of frictional heat developed.

It is, therefore, an object of the invention to provide an article adapted to withstand severe wearing conditions imposed by high frictional forces and high temperatures at the outer surface thereof.

A further object is to provide a relatively thin walled coated article having an outer protected surface exposed to a high velocity and high temperature resulting from the frictional heat caused by re-entry into the earths atomsphere.

A still further object is to provide a high temperature resistant air craft component having an outer surface exposed to atmospheric friction, said surface comprising an ablatable layer including a high melting continuous phase, and second, less refractory phase.

In the sole figure of the drawings the white area is the boron and the grey area is zirconia and the dark areas are voids.

In brief, the invention contemplates a relatively thin walled, formed body or article which is provided on its outer surface with a thin lamellar layer, said layer being bonded to the article surface and comprising microscopic leaves which are disposed in overlapping, and interlocking relationship so as to be bonded to each other and to the said surface. The leaves comprising said layer are formed from comminuted particles of a coating com-v position made up of Zirconia and boron, which particles are propelled at high velocity by a hot gas stream against the surface to be coated. The particles are thereby de formed into a flattened leaf-like configuration.

The presently disclosed coating may be applied to the substrate material or article by any of sevral processes generally known to the art. For example, methods found to be suitable in forming the coating are fully described and claimed in US. Patent Nos. 2,714,563, and 2,861,- 900. These methods include the detonation process and the jet plating process respectively. It has also been found that plasma jet coating process described in co-.

pending application Serial No. 850,444 filed on Novembet 2, 1959, now Patent No. 3,016,447, by R. M. Gage et al. may be ultilized.

To briefly and generically describe the aforementioned coating methods, an apparatus or gun is provided for receiving a highly detontable gaseous mixture. As the mixture is ignited and the resulting gases caused to, expand in a substantially. confined chamber, the coating composition in finely powdered form may be introduced to the chamber or else into the detonation stream directly. The chamber is provided with an egress passage leading into an elongated narrow barrel. As the hot expanding gases pass from the chamber and into the gun barrel at a high velocity, the particles of coating ma-.

becoming bonded to the substrate surface and mutually.

to each other. Successive detonations within the coating apparatus accompanied by successive additions of coating material cause a gradual build up of irregularlyshaped and interlocked leaves into a composite lamellar layer.

In the jet plasma process mentioned above, a gas is also used as a heating and carrier medium. The gas in this instance though, need not be detonatable, it may be, an inert gas such as argon. A stream of the gas is heated to a high temperature by being passed between electric arc forming electrodes, and is thence constricted in a narrow passage to a flow having the required velocity. As in the previously described processes, particles of the coating metal are then injected into the rapidly flowing, heated stream to be impinged against the work piece surface. The particles on striking said surface in a heat softened condition, deform and become bonded to each other and to the said surface in a lamellar layer.

The coating may be applied satisfactorily to various substrate surfaces so long as the surface is able to withstand the high temperature gas stream and is adapted to eceive the coating material. As mentioned previously, this material may be stainless steel, aluminum, or titanium which have been formed into the desired shape.

The thermally protected article contemplated by the present invention consists of a thin walled body as for example a missile nose cone, said body having bonded to the outer surface thereof a zirconia-boron composite layer in which the zirconia portion constitutes between 50-70 percent by volume, the remainder being boron. It is generally recognized in the art that during the course of applying a coating in one of the above-discussed methods, the finished coating composition as completed varies somewhat from the composition of the starting material due to changes or volatization which occurred during the coating process. For example, it has been found that for coatings having a 60 volume percent zirconia mixture applied by the above-mentioned detonation plating process are found to have a final composition of about 70 volume percent zirconia and volume percent boron. When on the other hand, the jet piasma method is employed, the finished coating will usually have substantially the same composition as the starting powder mixture.

In an example of a test article made by the abovediscussed process, a powder mixture of 60 volume percent zirconia (325 mesh) and volume percent boron (100 mesh), was introduced to a detonation plating apparatus employing oxygen and acetylene as the components of the detonatable gas mixture. This mixture was intermittently fired and the resulting hot, high vel'ocity, powder-gas efiluent was directed against the fiat surface of a fii-in. x 2-in. x 2-in. copper plate to form a A in. thick zirconia-boron coating. The resulting coating density on examination was determined to be 4.1 grams/cm. V

The coated test piece was then disposed at a angle and subjected to impingement thereon of the hot, high velocity gas flow from a plasma jet testing device under heat flux conditions of about 1000 B.t.u./ft. sec. for 10 seconds. The gas jet was about /2-in. dia. and the coated test piece was positioned l-inch from the discharge end of the arc device. These test conditions were established to simulate friction heating of a coated missile component while re-entering the earths atmosphere. Subsequent examination of the test piece indicated that there was no gross spalling of the coating nor separation from the baseplate, which result indicated good resistance to thermal shock. It was observed that while the front face temperature of the coated plate averaged about 2280 C. during the test, the back face temperature rise was only about 60 C. thus indicating a rather low coefficient of thermal conductivity of the coating. For test purposes, the front face temperature was measured by an optical pyrometer while the back face temperature was measured by a thermocouple embedded in the base material near the coating interface. In this test, ablation protection for the base material was indicated by a coating volume loss of about 1.2 cm. during the 10 sec. test.

The ability of the present novel coating to withstand damage at high heat flux conditions and thus protect the base material is believed to be due at least in part to the unique combination in the layer of a high melting point and being a good thermal emitter so as to radiate heat away from the coated body.

Also the ability of the coating to ablate away dissipates a great deal of heat which would ordinarily get to the outer surface. While it is believed that useful ablation coatings could be prepared from coating mixtures containing -65 volume percent zirconia and 35-50 volume percent boron, the above-mentioned volume percent zirconia 40 volume percent boron is preferred. The physical condition of the boron used will also have an effect on the composition range. For example, a powdered coating composition must be capable of being readily dispensed into a coating device. It was found though that about 35 to 40 volume percent amorphous boron can be used while 35 to 50 volume percent is useful when the boron is crystalline.

What is claimed is:

1. A high temperature resistant article characterized by thermal and shock resistance to a high velocity stream of fluid passing along the outer surface thereof and being ablatable under such conditions, said article comprising a thin walled metallic body having an external surface, said surface being provided with a coating layer having a lamellar structure consisting of microscopic leaves disposed in overlapping, and interlocking relationship, said leaves being bonded to each other and to the surface article without substantial alloying at the article-coatinginterface, said coating layer consisting of mutually bonded and intermixed leaf-like particles of zirconia, and boron, the zirconia leaves being present in a range of from 50 to 70 percent by volume of the coating layer, the remainder of said particles being boron.

2. A laminated produce of manufacture characterized by thermal and shock resistance to high velocity streams contacting the outer surface thereof, and being ablatable under such conditions, said article comprising a thin metallic wall defining the substrate of said laminated product, a coating bonded to said substrate to form a shock resistant thermal shield, said coating comprising a lamellar structure consisting of microscopic leaf-like particles disposed in overlapping and interlocking relationship to cover said surface, said leaf-like particles being bonded to each other and to the substrate without substantial alloying at the interface thereof to form a layer having a density of about 4.1 grams of said coating per cubic centimeter, the proportional amounts of said leaves in the coating being about 50 to 70 percent by volume of zirconia, the remainder being boron.

3. An air craft outer surface component characterized by high thermal and shock resistance when subjected to a high velocity flow of air along the surface thereof, said product comprising a formed thin-walled article fabricated from a metal normally unable to withstand the heat of friction caused by contact of said component with atmospheric air, the outer surface of said article being provided with a protective coating bonded thereto, said coating comprising a lamellar layer of microscopic leaflike particles disposed in overlapping and interlocking relationship to cover said surface, said leaves being mutually bonded to each other to form a coating, said particles being present in proportions of about 60 parts by volume of zirconia to about 40 parts by volume of boron.

4. An article of manufacture substantially as described in claim 3 wherein the protective coating has a density of about 3.8 grams of coating per cubic centimeter.

5. A coating composition adapted to be applied to a metallic substrate surface which comprises; comminuted particles in a mixture comprising between 50 and 70 percent by volume of zirconia, the remainder being boron.

6. A coating composition as described in claim 5 comprising about 60 volume percent zirconia, and about 40 volume percent boron.

7. A wear-resistant coating on a metallic work piece surface which comprises a lamellar layer consisting of microscopic leaves disposed on said surface in dispersed order and in overlapping and interlocking relationship as to be bonded to each other and to the surface, said layer consisting of from 50 to 70 percent by volume of zirconia, the remainder being boron.

8. A coating substantially as described in claim 7,

wherein the zirconia leaves are present in the layer in an amount of about 60 percent by volume, the remainder of the leaves being boron.

9. Method of applying a Wear-resistant coating to a metallic surface which comprises; providing a metallic surface to be coated, providing a device for discharging a heated flow of gas, introducing to said device a flow of the gas, heating said gaseous flow to an elevated temperature and directing the flow through a constricted passage to obtain a high velocity stream, injecting into said heated high velocity stream a powdered coating mixture having 5 6 in the composition thereof an amount of zirconia particles 2,861,900 11/1958 Smith et a1. 117-105 between about 50 to 70 percent volume of the mixture, 3,016,311 1/1962 Stackhouse 117l05 the remainder being boron, and impinging said particles 3,016,447 1/1962 Gage et al. 21976 against the metallic surface to be coated whereby the 3,054,694 9/1962 Aves 117 71X heated particles may strike said metallic surface and be 5 deformed there against to form a lamellar, leaf-like layer OTHER REFERENCES of said particles bonded to each other and to said metallic Ingham et all Metallizing Handbook, VOlS- I and surface. pp. B15 and B-55, Metallizing Engineering Co,, West- References Cited by the Examiner y, 1959- UNITED STATES PATENTS WILLIAM D. MARTIN, Primary Examiner. 2,714,563 8/1955 Poorman et al. 117-105 2,822,302 2/1958 McCaughna 117 221 RICHARD D-NEVIUSEmmme" 

1. A HIGH TEMPERATURE RESISTANT ARTICLE CHARACTERIZED BY THERMAL AND SHOCK RESISTANCE TO A HIGH VELOCITY STREAM OF FLUID PASSING ALONG THE OUTER SURFACE THEREOF AND BEING ABLATABLE UNDER SUCH CONDITIONS, SAID ARTICLE COMPRISING A THIN WALLED METALLIC BODY HAVING AN EXTERNAL SURFACE, SAID SURFACE BEING PROVIDED WITH A COATING LAYER HAVING A LAMELLAR STRUCTURE CONSISTING OF MICROCOPIC LEAVES DISPOSED IN OVERLAPPING, AND INTERLOCKING RELATIONSHIP, SAID LEAVES BEING BONDED TO EACH OTHER AND TO THE SURFACE ARTICLE WITHOUT SUBSTANTIAL ALLOYING AT THE ARTICLE-COATINGINTERFACE, SAID COATING LAYER CONSISTING OF MUTUALLY BONDED AND INTERMIXED LEAF-LIKE PARTICLES OF ZIRCONIA, AND BORON, THE ZIRCONIA LEAVES BEING PRESENT IN A RANGE OF FROM 50 TO 70 PERCENT BY VOLUME OF THE COATING LAYER, THE REMAINDER OF SAID PARTICLES BEING BORON. 